He has a reason to join, a step in the right direction for the storytelling, but the reasons itself are shaky and flimsy at least.
He joins to steal souls away to the Shadow Isles:
That'll be hard considering that the League won't let him do that out the Fields. Doing it in the Fields won't amount to anything.
He wants to break the sanity and spirit of the champions
Somewhat more sensible than the previous, but not by much. A champion has to endure being shot, burned, devoured by sharks/trolls/void creatures, cut, incinerated, mauled, blown up, poisoned, subject to fear/nightmares/visions of their demise, turned to stone, hit by infected cleavers, decapitated etc... every time they're summoned.
And that was before Thresh came out. What's a guy with a chain and spooky powers got that makes all the others previously mentioned lesser in comparison?